People throughout our society have become increasingly aware and concerned about the environmental issues that plague the world. The depletion of the ozone layer, the rain forests, and clean water are just a few of the environmental issues that are being addressed. One approach in addressing these issues includes preserving resources by recycling them. Consequently, the recycling industry has become instrumental to serving this need.
In the recycling field, one area of increasing interest is the reuse of wastepaper. Millions of tons of wastepaper are generated every year in the United States. Recycling this wastepaper can save countless trees, as well as provide other ecological and economic benefits. However, the key to reuse of this wastepaper is the removal of contaminants from the wastepaper, thereby facilitating the use of recycled or secondary fibers from the wastepaper.
The paper recycling industry encounters a variety of contaminants in wastepaper. Many of these contaminants adhere to paper fibers and therefore may cause problems during the recycling process. One such contaminant is “stickies”, which were used originally as paper adhesives or tacky adhesives. Stickies typically are classified as hot melts, pressure-sensitive adhesives (PSAs), latexes, and binders. Pitch is another contaminant associated with both virgin and secondary fibers. Pitch is a part of the extractives from wood, and is released during pulping.
Contaminants may cause operational and product quality problems. Specifically, contaminants may be deposited on wires, felts, press rolls, and drying cylinders of paper machines. In addition, contaminants may hinder bonding of fibers, increase web breaks, and reduce product quality in the papermaking process. Consequently, contaminants must be controlled in order to improve papermaking operations and product quality.
Tack is the sticky property of paper adhesives, paperboard adhesives and glue coating materials. The tack of an adhesive and the adhesive's ability to bond to another surface is dependent, in part, upon the surface energy of the adhesive. Reducing the tack of contaminants can minimize the propensity of the contaminants to attach to paper machine surfaces, thereby leading to fewer operational problems.
Various prior art methods are used to reduce the tack of the contaminants. Some methods use repulpable or recyclable adhesives. More common methods include chemical additives for modification, detackification, or pacification of the contaminants. For instance, detackification of contaminants is frequently accomplished by adding minerals, such as talc, or surface-active chemicals. These minerals and surface-active chemicals attach to the surface of the contaminants and alter their surface properties, thereby causing tack reduction. This method of tack reduction is described in a publication entitled, “Successful Approach in Avoiding Stickies,” by S. Abraham, Tappi J., 81:2 79-84 (1998), which is incorporated herein by reference. Nonetheless, chemical additives can be very expensive and may cause other problems in the papermaking process, such as a decline in product quality.
Mechanical methods for controlling contaminants include dispersion, screening and cleaning. Dispersion is a technique by which contaminants are broken up into smaller and smaller particles until they are invisible in the final product. Unfortunately, the overall appearance of a product may be diminished greatly by the presence of contaminants. In addition, when the product containing contaminants is wound, sticking may occur between adjacent layers.
Screens and centrifugal cleaners are typically used to remove stickies, pitch and debris from the fiber stream. In general, screens are used to physically separate fiber from contaminants based on the differences between the sizes and shapes of contaminants and the holes or slots in the screen. One problem is that screens cannot remove contaminants that are either smaller than the size of the screen hole or deformable enough to pass through the screen hole. Centrifugal cleaners separate contaminants from fiber primarily on differences between the specific gravities of the fiber and the contaminant. However, separation is poor if the specific gravity of the contaminant is similar to the specific gravity of the fiber.
Therefore, there is a need for a system and method for improving the removal efficiency of contaminants, such as stickies and pitch, from a fiber stream. In addition, there is a need for a system and method that can detackify contaminants by altering, without the use of chemicals, the surface properties of the contaminants. There is yet another need for a system and method that alters the tack of materials inexpensively and simply.